Based on the popularity of my LED Interior Lighting post, and by popular request in the comments section, I decided to write another guide for LED License plate bulbs on a B7 Audi A4, S4 and RS4. They definitely look awesome, and are a relatively cheap mod at ~$30 total. The trouble with this mod is that the with some B7 models (any A4, S4 or RS4 from 2005.5-2008), the installation of these bulbs sometimes causes a Light Out error on your dash even if you buy “Error Free” bulbs, which gets super annoying very quickly. There are many theories as to which models or trim packages throw the Bulb Out error and which don’t, but from everything I’ve read, it’s mostly just luck of the draw/random. Many believe that if you have the multi-color information display in your gauge cluster (the LCD screen between your speedometer and RPMs) you are more likely to have an bulb out warning message, although Bulb Out errors do occur for the non-color version (those with only red colored pixels only).
Regardless – you have two options…you can try to buy “Error Free” bulbs and see if they actually work (below, left), or you can buy “normal” LED bulbs (below, middle) and install resistors (below, right). The advantage of buying non-error free bulbs is that they’re brighter, and they don’t have the built in resistor which can cause the bulbs to get hot and eventually burn out quicker or cause other issues – you also don’t have to worry about matching resistors and worrying about sending too much current with the built-in resistor plus add-on resistor. As you’ll see in the Amazon reviews the error-free bulbs still have a high rate of incidents, so personally I would consider picking up the non-error-free bulbs and resistors.
Bulb Installation: lift up your trunk lid, remove your license plate for a little extra wiggle room, then remove the two screws holding the light cover in. You’ll probably need a specialty screwdriver like the ones used on eyeglasses as its a very small screw. Then pull out the old bulbs and snap in the new bulbs. Be sure to test it before re-installing the light covers, just to make sure the bulbs are making contact (you may need to bend the contact points of the base) as well as to see if you get any bulb out errors first.
Resistor Installation: if you need to install resistors, you’ll need to remove the liner of your trunk to access the wires. There are two screws that come out using a Phillips screwdriver, pictured below. One is below the trunk release handle and the second one is in a similar location on the opposite side of the trunk. Once the screws are removed, then there are four clips on the top edge and four clips on the bottom edge which will release when you firmly tug near each clip.
Once the trunk liner is pushed to the side, install the resistor. You’ll want to tap the brown wire and the purplish-gray wires that are furthest to the left of the harness. I found easiest to remove the harness to install the resistor first.
I find it works best to strip part of the brown wire and gray/purple wire first so the resistor makes good contact, as I find the red-plastic splicers don’t always cut all the way through. It doesn’t matter which wire you attach to which, just make sure the resistor bridges the brown wire and gray/purple one. Warning: the resistor gets VERY hot VERY quickly so don’t touch it once you’ve turned the lights on. I mounted the resistor using 3M double sided tape and then plugged back in the harness and turned the car on to test that the lights still turn on and no error warnings – success! If you still get a warning, go back and check our connections and make sure the exposed wires are lined up correctly within the red plastic splicer.
Disclaimer: I’m not responsible if you buy any bulbs and they don’t work, nor any damage you may cause to your car while working on it. Any modifications you perform on your vehicle are done at your own risk. That being said, there isn’t too much of a risk of causing any damage here, except you may be out the cost of LED bulbs in the event they flicker or cause a bulb out message.
Bonus – while your’e at it, consider doing the Reverse Light LED mod to complete the look: